Timing of the global Dasberg Crisis - implications for Famennian eustasy and chronostratigraphy.

Hartenfels Sven, Becker Ralph Thomas

Zusammenfassung

The Dasberg Crisis is characterized by global extinctions of pelagic faunas, especially of ammonoids, and by sudden facies changes, such as black shale intercalations, at the Hembergian/Dasbergian (Upper Devonian = UD IV/V) and near the Lower/Middle expansa Zone boundaries. It can be recognized globally in the pantropical belt, from North America, to NW Africa, Europe, Asia, and NW Australia. Detailed investigations of sections in the Rhenish Slate Mountains, Carnic Alps, and SE Morocco showed that black shales and limestones consistently have a different age within the Endosiphonites Zone (UD V-A1) in Europe and Morocco. Hypoxic crisis beds of the northern Rhenish Massif and the Carnic Alps fall in the top of the Lower expansa Zone, within the highest range of Polygnathus styriacus; those of the Tafilalt (SE Anti-Atlas) in the basal Middle expansa Zone, within the lowest range of Bispathodus aculeatus aculeatus. Typical last UD IV ammonoids (Prionoceras) may co-occur in Moroccan crisis beds together with a range of UD V index forms, which gives evidence for a stepwise extinction. The rarity of Palmatolepis gracilis expansa requires recognition of the Lower expansa Zone with the help of alternative zonal markers, such as Pa. rugosa rugosa or B. stabilis Morphotype 2. There is no evidence for a significant eustatic transgression at the base of the Lower expansa Zone, which lies well within successions of Platyclymenia faunas. All these aspects strongly disqualify the use of Pa. gracilis expansa for formal definition of an Upper Famennian substage. The diachronous nature of Dasberg Crisis beds and the locally irregular entry of B. aculeatus aculeatus also speak against using the crisis onset or the base of the Middle expansa Zone for chronostratigraphic definition. The eustatic sea-level curve of Johnson et al. (1985) needs to be revised within the T-R Cycle IIf.